MFC r355182: Fix use-after-free in case of L2ARC prefetch failure.

MFC r355182: Fix use-after-free in case of L2ARC prefetch failure.

In case L2ARC read failed, l2arc_read_done() creates _different_ ZIO
to read data from the original storage device.  Unfortunately pointer
to the failed ZIO remains in hdr->b_l1hdr.b_acb->acb_zio_head, and if
some other read try to bump the ZIO priority, it will crash.

The problem is reproducible by corrupting L2ARC content and reading
some data with prefetch if l2arc_noprefetch tunable is changed to 0.
With the default setting the issue is probably not reproducible now.